1. Field of the Invention
The present invention relates generally to insulated electrical conductors and, in particular, to a method for making abrasion resistant ignition wires for spark plugs and similar ignition devices.
2. Related Art
Ignition wires for spark plugs commonly include a conductive core, a conductor or semi-conductor coating layer, a release coating, an electrical insulation layer, a strengthening layer to provide enhanced tensile strength and an insulating outer sheath or jacket, and may also include various adhesive and other inner layers. The outer jacket also frequently includes a cosmetic or decorative layer or printed portion on its outer surface. The layers described above may be arranged in different combinations and orders and their particular arrangement will vary depending on the intended application environment and other factors.
In addition to various solid or stranded metal wire configurations, the conductive core may also comprise a wire-wound configuration. Wire-wound cores typically have a braided or otherwise woven insulating core member, such as a rope, to provide tensile strength. This member may have a ferrite or other coating applied that is in turn wrapped with a metal wire and coated with an adhesive and/or conductive or semi-conductive layer. Examples of such wire wound conductive cores are taught by Miyamoto et al. in U.S. Pat. No. 4,435,692 and Coffe et al. in U.S. Pat. No. 4,700,171. Miyamoto et al. teach a wire wound ignition cable in which a resistance wire is wound over a woven member, such as a woven aramid string, which has been coated with a chlorinated polyethylene/ferrite mixture to provide the conductive core. The resistance wire and ferrite coated core are in turn coated by a woven strengthening member and an extruded sheath of a blend of polyethylene and ethylene propylene diene monomer (EPDM). Coffe et al. discloses an ignition cable similar in some respects to that taught by Miyamoto et al. in which a conductive core is formed by dip coating a strengthening member formed from a glass fiber bundle with an insulating layer containing a mixture of EPDM and magnetic particles, such as iron oxide. The coated woven core is then helically wrapped with a resistance wire conductor, such as various Ni alloys. The resistance wire of Coffe et al. is then dip coated with a semi-conductive thermoplastic polymer, such as a silicone or acrylic polymer. The semi-conductive thermoplastic polymer contains carbon particles and release agents which allow subsequently applied insulating layers to be stripped away cleanly. The conductive core of Coffe et al. is coated with an insulating layer formed from EDPM, an optional fiberglass braid layer, and a polymer jacket formed from a mixture of EPDM, ethylene vinyl acetate copolymer, phenolic antioxidant and a metal salt antioxidant. In general, for conductive cores as described in Miyamoto et al. and Coffe et al., a release coating is applied to the exterior of the core or incorporated into a conductive coating in order to promote removal of the insulating layer from the core during the application of terminations or connectors to the ignition wire.
The ignition wire insulation layer may be made from various materials that provide electrical insulation and are resistant to degradation at the elevated operating temperatures of an internal combustion engine. Examples of materials that have been used for the insulating layer in various wire configurations include EPDM and various silicones.
The strengthening layer is typically made from fiberglass and comprises a woven sheath. This layer may be woven over the insulation layer directly or pre-woven and applied over the insulation layer.
The insulating jacket is typically made of a material that is resistant to high levels of heat, as well as abrasion, because it forms the outer wall of the ignition wire. Various materials have been used for the insulating jacket, such as EPDM, various silicones and other materials, depending on the intended application and other factors. The jacket is typically extruded over the insulating layer. The process of extrusion can alter the thermal, mechanical and/or chemical properties of both the jacket and the underlying insulation layer. As such, the overall appearance of the outer surface of the jacket may be affected, as well as the abrasion resistance of the jacket itself.
As noted above, in many ignition wire applications, it is desirable to apply cosmetic or decorative materials, such as various inks and the like, which are used to print information on the wire such as the manufacturer's name, product numbers, wire sizes, manufacturer's logos or trademarks, performance characteristics, specifications, or other important information, as well as ornamental designs. Being located on the outer surface, such materials are subject to high temperatures, abrasion from dirt and other under-hood sources of abrasion, mechanical stress, chemicals and other agents which promote their degradation.
To improve the mechanical, thermal and chemical properties of the insulated wires, the jacket may include a coating material coated thereon. U.S. Pat. No. 4,000,362 to Kawaguchi et al. discloses an electrical insulated metallic wire comprising a releasing layer coated on the metallic wire with a baked-on insulating layer superposed on the releasing layer. The insulating layer is formed on the releasing layer by coating and baking a silicone-containing insulating varnish having a releasing ability on the releasing layer. The releasing ability of the silicone-containing insulating varnish allows the insulating layer to be easily stripped from the releasing layer. It is believed that the construction of Kawaguchi would not be applicable for the protection of the cosmetic or decorative materials or enhancing the abrasion resistance of the jacket because it is applied over a release coating and is designed to be readily removed from the outer surface of the wire jacket.
Therefore, it is desirable to develop ignition wires with improved protection, such as improved heat and abrasion resistance, to the outer surface of the jacket. Further, it is also desirable to develop ignition wires with enhanced protection for cosmetic or decorative materials, such as inks, that are applied to the outer surface of the wire jacket.